An atom consists of a positively charged nucleus surrounded by negatively charged electrons.
The nucleus contains protons, each of which has a charge of +1, and neutrons, which are uncharged.
For a pure element, the number of protons and electrons are equal. All atoms of an element contain the
same number of protons (and electrons). Different isotopes of an element contain different numbers of
neutrons.

The atomic weight of an isotope is the sum of the numbers of protons and neutrons and is
denoted as a left superscript. The naturally occurring isotopes of N are 14N (7 protons and 7 neutrons)
and 15N (7 protons and 8 neutrons) (Cotton and Wilkinson 1972). The atomic weight of an element is a weighted average of the
atomic weights of the naturally occurring isotopes of the element. The (statistical) weights are the natural
abundances of the isotopes.

Some isotopes spontaneously decay to form isotopes of different elements that are more stable. The decay of these
isotopes is accompanied by the emission of charged particles (alpha, beta) and/or light (gamma rays).
Isotopes that spontaneously decay are called radioactive isotopes. Isotopes that do not spontaneously
decay are called stable isotopes. Both naturally occurring isotopes of N are stable.